<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>结果:</div>
<script>
  let i1 = document.querySelector("#i1");
  let i2 = document.querySelector("#i2");
  let d = document.querySelector("div");
  function f(x) {
    if (isNaN(i1.value)||isNaN(i2.value)){
      d.innerText = "输入错误!";
      return;
    }
    switch (x) {
        case 1: //+
            d.innerText = "结果:" + (i1.value*1+i2.value*1);
            break;
        case 2: //-
            d.innerText = "结果:" + (i1.value-i2.value);
            break;
        case 3: //*
            d.innerText = "结果:" + (i1.value*i2.value);
            break;
        case 4: // /
            d.innerText = "结果:" + (i1.value/i2.value);
            break;
    }
  }

</script>
</body>
</html>